Synthesis, crystal structure and conductive properties of garnet-type lithium ion conductor Al-free Li_7−xLa₃Zr_2−xTa_xO₁₂ (0 ≤ x ≤ 0.6)

抄録

We investigated the phase formation and the conductive properties of the Al-free Li_7−xLa₃Zr_2−xTa_xO₁₂ (0 ≤ x ≤ 0.6) samples. The X-ray diffraction patterns of the Li₇La₃Zr₂O₁₂ (x = 0) and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (x = 0.6) samples were assigned to be single phases of tetragonal (space group: I4₁/acd) and cubic (space group: Ia-3d) structures, respectively. On the other hand, the intermediate compositional samples of Li_7−xLa₃Zr_2−xTa_xO₁₂ (0.2 ≤ x ≤ 0.5) showed a coexistence of both the tetragonal and cubic phases. To investigate the conductive property of the prepared samples, the Li-ion conductivity was measured in a temperature range from 253 to 313 K by AC impedance method. All of the Al-free Li_7−xLa₃Zr_2−xTa_xO₁₂ (0 ≤ x ≤ 0.6) samples exhibited relatively high conductivity of ∼10⁻⁴ S cm⁻¹ at room temperature, and the Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (x = 0.5) sample showed the highest Li-ion conductivity of 8.4 × 10⁻⁴ S cm⁻¹ at room temperature. In order to clarify the relationship between the Li-ion conductivity and the Li-ion arrangement, the crystal structure analysis of Li_7−xLa₃Zr_2−xTa_xO₁₂ (0 ≤ x ≤ 0.6) was performed by Rietveld analysis using powder X-ray diffraction data. The Li(2) atom at 96h site was gradually shifted together with increasing Ta-content from x = 0.2 to 0.5 resulting the shorter Li–Li distance in the loop structure of the cubic garnet-type framework structure.